CFG: Delay creating Dtors for CompoundStmts which end in ReturnStmt

Summary:
VisitReturnStmt would create a new block with including Dtors, so the Dtors created
in VisitCompoundStmts would be in an unreachable block.

Example:

struct S {
  ~S();
};

void f()
{
  S s;
  return;
}

void g()
{
  S s;
}

Before this patch, f has one additional unreachable block containing just the
destructor of S. With this patch, both f and g have the same blocks.

Reviewers: krememek

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D13973

llvm-svn: 253107